The United States should seize the initiative and soon mount a massive experiment to search for a hypothesized type of nuclear decay that is possible only if an elusive, nearly massless particle called the neutrino is—weirdly—its own antiparticle. That’s one of four recommendations in a new long-range plan developed by U.S. nuclear physicists. The plan, presented to a federal advisory panel today in Washington, D.C., will inform planning for the coming decade in the Department of Energy’s (DOE’s) nuclear physics program, and the National Science Foundation’s (NSF’s) physics program. If researchers observe the new decay—and they hope to start work on the experiment within 3 years—the discovery would require rewrites of textbooks in nuclear and particle physics.
The report, which was unanimously approved by the Nuclear Science Advisory Committee (NSAC), immediately met with praise from DOE officials. “It’s an ambitious plan,” says Patricia Dehmer, acting director of DOE’s $5.1 billion Office of Science, which will spend $596 million this year on its nuclear physics program. “It builds on the past and looks to a very promising future.”
As expected, the plan also recommended that U.S. nuclear physicists eventually build a new collider, one that would smash a beam of electrons into a beam of protons or heavier atomic nuclei. But the report put no firm timeline on when such an electron-ion collider could be built, and suggested that it could not be completed until the end of the 2020s at the earliest. Still, physicists and DOE officials say that the recommendation—which ranked third in the report—is still notable, because it shows that the U.S. community has come together behind the concept of building such a collider. “I think it’s defining the goal,” Dehmer says.
The report’s top recommendation, however, is for researchers to first fully exploit the three major facilities U.S. nuclear physicists already have. Physicists at Thomas Jefferson National Accelerator Facility in Newport News, Virginia, are completing a $338 million upgrade to their Continuous Beam Electron Accelerator Facility (CEBAF), which they use primarily to probe the internal structure of protons and neutrons. Physicists at Michigan State University in East Lansing are building the $730 million Facility for Rare Isotope Beams (FRIB), a linear accelerator that, when it is completed in 2022, will generate exotic nuclei and study their structure. Finally, since 2000, physicists at Brookhaven National Laboratory in Upton, New York, have used their Relativistic Heavy Ion Collider (RHIC) to smash nuclei such as gold together and literally melt the protons and neutrons into an amorphous plasma of their constituents—particles called quarks and gluons—like that that filled the newborn universe.
The new long-range plan calls for running all three facilities for the foreseeable future, even RHIC, which is arguably closest to the end of its life. “What we’re really saying is that we want to run RHIC for another 5 to 7 years,” says Donald Geesaman, a physicist at Argonne National Laboratory in Illinois and chair of NSAC.
Just 2 years ago, it seemed unlikely that U.S. physicists would be able to run all three of the current facilities. In 2012, facing the prospect of extremely tight budget, DOE tasked NSAC with deciding which facility, CEBAF or RHIC, it would sacrifice if it had to. The following January, physicists reluctantly decided that if forced to chose, they would opt to shut down RHIC. However, since then, DOE’s nuclear physics budget has rebounded sufficiently that now running the three facilities in concert is feasible, even if the budget grows only with inflation over the next several years, the report says. Those budget increases reflect how effectively the community presented its case for avoiding such cuts, says Timothy Hallman, DOE’s associate director for nuclear physics.
The report’s fourth recommendation is to invest in more small- and mid-scale projects, which have gotten short shrift in recent years. “That was the right thing to do to get FRIB built,” Geesaman says, but now it’s time to correct course.
Clearly the newest element in the long range plan is the call to move quickly on the search for the rare nuclear decay, which is called neutrinoless double beta decay. “It’s certainly putting [the project] in a different category of probability than it has been in up to this point,” Hallman says.
In ordinary beta decay, a neutron in a nucleus such as tritium can change into a proton by spitting out an electron and an anti-neutrino. Some nuclei, such as selenium-82, can spit out two electrons and two antineutrinos in so-called double beta decay. But in neutrinoless double beta decay, only two electrons would come out of a nucleus. For that to happen, the neutrino would have to be its own antiparticle, as the antineutrino emitted with one electron would instantly be reabsorbed as a neutrino to trigger emission of the second electron.
If the neutrino were its own antiparticle, it would be the only building block of matter with that property—although force-carrying particles like the photon gluon are their own antiparticles. It would also mean that the neutrino would have to get its mass in away different from other matter particles, that are weighed down by the so-called Higgs mechanism.
To spot such rare decay—if it exists—physicists need to work far underground, where background radiation is low, and to observe large amount of nuclei, such as xenon-136, germanium-76, or tellurium-130. Physicists around the world are already working on experiments using several kilogram of such material. But spotting the decay will likely take a tonne-scale experiment, and the report calls on the U.S. to start building one as soon as 2018. “The neutrinoless double beta decay arena is very competitive internationally,” says Robert McKeown, a physicists at Jefferson Lab. “If the U.S. wants to lead we can’t wait.” A tonne-scale experiment is likely to cost a few hundred million dollars.
Fulfilling the plan may not be easy. It assumes the nuclear physics budget will increase by 1.6% above inflation each of the next 10 years—or at an absolute rate of between 3.5% and 4%. That’s a tall order, Dehmer cautioned at the advisory panel meeting. Still, she noted, the nuclear physics community has done about that well since it presented its last long-range plan in 2007. Dehmer credited that budgetary success in part to the community’s willingness to embrace such plans—something that physicists in other field sometimes struggle to do.